In the context of road safety, for example, numerous fatal accidents are caused by vehicles running off the road. It is therefore important to have continuous awareness of the position of the vehicle, regardless of the environmental or climatic conditions. Prevention of running off the road can be further increased if information on the infrastructure (signaling, radius of curvature, camber, etc.) is supplied to the vehicle.
Numerous guidance and information transmission systems already exist. Of these, the choice of a magnetic detection system yields many advantages. This is because the magnetic properties of materials are practically unchanged by external climatic conditions such as rain, mist, brightness, etc. A system of communication between a mobile element and an infrastructure based on magnetism is also independent of the external environment in the form of tunnels, valleys, and the like.
There are known information systems based on magnetic methods, which use magnetic marking in the form of permanent magnets. In this case, the magnets generally serve only as positioning references. Their use for encoding information can only be limited. This is because the information is fixed and can only be modified by replacing a magnet having a certain polarity with another magnet of different polarity. The cost of modifying the information encoded by the permanent elements is therefore high. The magnetic detection system fitted in a vehicle has to detect a magnetic field whose attenuation is proportional to the cube of the distance between the permanent magnet and the magnetic sensor. Consequently, the magnetic field becomes very weak when the distance between the emitter and receiver increases, leading to poor performance of the positioning system.
The use of magnetic marking in the form of a magnetic strip deposited on or in the infrastructure is a helpful alternative. This is because the magnetic field can be inversely proportional to the square of the distance between the strip and the on-board sensor, rather than to the cube as in the case of individual permanent magnets.
U.S. Pat. No. 6,289,269 B1 discloses a system for guiding a vehicle on an infrastructure, comprising a continuous guide in the form of a magnetic strip applied to the surface of this infrastructure. A double vertical magnetic sensor is used to measure the magnetic field of the strip and the surrounding magnetic field in its lower part, and the surrounding magnetic field only in its upper part which is farther from the magnetic strip. The position of the vehicle is then deduced from the difference between these two measured magnetic fields. However, this method cannot provide a precise value of the positioning of the vehicle on the infrastructure. This is because the proposed double sensor does not take into account the variation of the surrounding magnetic field or the influence of metallic bodies. Furthermore, the magnetic strip is used only for guiding the vehicle.